In general, photosensitive printing plates are classified as planographic plates, intaglio plates and relief plates. The photosensitive planographic printing plate is produced by rendering the surface of a support hydrophilic by treating the surface either chemically or physically or by coating a hydrophilic polymer on the surface, followed by applying a suitable photosensitive material on the thus prepared hydrophilic surface.
Typical surface treatments include mechanical surface treating such as brush graining, or chemical surface treating such as electrolytic graining and/or etching, and/or anodizing, and/or chemical surface treating which applies a further layer such as an alkali metal salt of phosphoric acid, a silicate, potassium fluorozirconate or anodic oxidation.
For a period of time, most lithographic plates were prepared from grained zinc plates which had been coated with a suitable photosensitive composition, dried, promptly exposed to secure the desired image, followed by applying a developing ink to the entire surface of the plate which was then washed with water to eliminate any water-soluble materials and developing ink. A gum arabic solution was thereafter applied to the printing surface of the plate to protect it until it was ready for use. The gum arabic provided chemical protection to the image and was easily washed off with water when it was desired to use the plate.
Jewett et al. in U.S. Pat. No. 2,714,066 describe a planographic printing plate formed from a thin metal sheet having at least one surface thereof treated to provide a tightly bonded, thin, preferably inorganic, hydrophilic surface treatment, formed from a solution of an alkali metal silicate, salicyclic acid, or other treating agent which will form a permanently hydrophilic scum-preventing and tone-reducing film overlaying and in firmly bonded contact with the surface of the plate, and having a coating of a light-sensitive organic material over the hydrophilically treated surface, i.e. over the surface of the scum-preventing and tone reducing film. The preferred substrate is an aluminum foil or sheet material which has been cleaned, for example, by immersion in a solution of trisodium phosphate.
The present invention is particularly concerned with presensitized plate systems in which the metal substrate has been prepared for application of the photosensitive material by anodizing. One problem which has historically plagued anodized presensitive plate systems has been background by resins, dyes, photosensitive materials, additives and the like. The natural porosity of the freshly anodized layers results in the absorption of materials of the photosensitive layer into the oxidized layer if the resultant layers are not sealed or interlayered during the manufacturing process with a suitable agent, for example, sodium silicate, rendering the area lipophilic causing ink to adhere to said absorbed material. However, it is well known that the organic nature of sensitizers, resins, additives and dyes gave rise to a shorter press life when such inorganic interlayers are employed. As a result, the use of silicate sealing or interlayering have generally been limited to medium and short run plates.
In order to minimize the background staining without adversely affecting press life, various approaches have been utilized by companies manufacturing anodized presensitized plates. These include the selection of dyes which visually do not appear to stain when examined by the naked eye, resin selection which will not permeate the anodizing pores but be lifted during developer treatment, etc. This approach, however, limits selection of the dyes, resins, photosensitive materials and additives, and may, on occasion, create differences in the ink/-water balance during a press run. Attempts to use lower concentrations of dyes have also been attempted but this generally limits the print out, between image and non-image areas, and contrast, between image and background areas. Another approach has been attempts to optimize anodizing conditions in order to avoid the porous layer formation. None of these compromises has been totally satisfactory.
One attempt to circumvent the various compromise approaches is to seal the anodized presensitized plate with an aqueous solution of polyvinyl phosphonic acid. This system retains the high print out and high contrast characteristics of the plate, generally eliminates the staining, and generally improves the image deletion, water/ink balance (press tinting), exposure, and shelf life. The press life, however, is about 25% reduced mainly due to sealing or interlayering chemicals with poor adhesion between anodic oxide and the coating in the image area.
It is accordingly the object of this invention to provide a new planographic printing plate in which the background dye staining problem is significantly overcome without substantially adversely affecting the press life and other desired characteristics of the planographic printing plate. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.